We spend more money prosecuting, convicting and incarcerating people charged with drug crimes, and it doesn't serve as a deterrent to future drug use or sales. That would cut several billion dollars from our annual budget if legalized marijuana and decriminalized possession of a small amount of substances like heroin, cocaine, and meth. Besides, if people want to screw up their lives by becoming addicts, what can the government do to stop them? Throw them in jail? It hardly works, since recidivism among drug users is really high.
Plus, think of all the uses marijuana has (smoking, baking, medicinal use, etc.). You have multiple sectors of the economy which could benefit tremendously from making marijuana legal. It could be the next IT revolution we need.